1. Field of the Invention
The present invention relates to an electronic component mounted on a substrate board through an array of solder bumps attached to the back of the electronic component so that the electronic component is electrically connected to the substrate board, especially a mounting structure of the electronic component on the substrate board. The present invention is suitably applied to a mounting structure of an electronic component used in portable electronic equipments.
2. Related Art
As shown in FIG. 16, a conventional ball-grid array (hereinafter referred to as BGA) package 101 has an array of solder bumps 102 on its back surface, and a multi-layer printed wiring board 103 has a plurality of substantially circular electrodes 104. Each of the solder bumps 102 is melted and connected to each of the electrodes 104 so that the BGA package 101 is mounted on the multi-layer printed wiring board 103.
As shown in FIG. 17, the multi-layer printed wiring board 103 has a lead wire 105 extending from each of the electrodes 104. The lead wires 105 which extend from most externally-located electrodes 104a (hereinafter referred to as external electrodes 104a) are generally formed on the surface layer of the multi-layer printed wiring board 103; thereby facilitating external electrical connection of the extending wires 105. The external electrodes 104a are to be connected to the most externally-located solder bumps 102.
The lead wire 105 is covered with solder resist 106 for the purpose of protection. In FIG. 17, a slant line portion indicates an area covered by the solder resist 106. The entire surface of the electrode 104 (104a) is exposed so that a contact area between the solder bump 102 and the electrode 104 is increased. Therefore, in the external electrode 104a, the lead wire 105 is partially exposed because the lead wire 105 is formed on the surface layer of the multi-layer printed wiring board 103, and the solder bump 102 is thereby connected to both the external electrode 104a and a part of the lead wire 105.
When an external shock is applied to the BGA package 101 or the multi-layer printed wiring board 103, stress is applied to the external electrodes 104a intensively, especially when the multi-layer printed wiring board 103 is made of flexible resin or the like. Therefore, bonding portion between the external electrode 104a and the solder bump 102 is required to have sufficient bonding strength to withstand this intensive stress.
As shown in FIG. 17, the lead wire 105 from the external electrode 104a generally extends toward outside by the shortest route. That is, when a polygon is formed by connecting each of the centers of the adjacent external electrodes 104a, the lead wire 105 extends from a portion of the external electrode 104a located outside the polygon, in a direction in which the lead wire 105 becomes away from a portion of the solder bump 102 located inside the polygon.
However, as shown in FIG. 16, the lead wire 105 extends from a portion A of the external electrodes 104a, which is located outside the polygon, and is mostly applied with the intensive stress when the external shock is applied to the BGA package 101 or the multi-layer printed wiring board 103. Therefore, when the external shock is applied, the solder bump 102 may be detached from the portion A, resulting in a contact failure between the BGA package 101 and the multi-layer printed wiring board 103.